This application claims the priority of Japanese Patent Applications No. 4-126185 and No. 4-126186 both filed May 19, 1992, which is incorporated herein by reference.
1. Field of the Invention
This invention relates to a protective apparatus for a power transistor for use with an audio frequency (low frequency) power amplifier circuit suitably applied, for example, for driving a loudspeaker for commercial use.
2. Description of the Prior Art
A protective apparatus for a power transistor is commonly employed in an audio frequency power amplifier circuit for driving a loudspeaker and prevents a power transistor from being broken down by an overload condition. An exemplary one of audio frequency amplifier circuits which include such protective apparatus for a power transistor is shown in FIG. 14. Referring to FIG. 14, the audio frequency amplifier circuit is generally denoted at 1 and includes a pair of power transistors Q.sub.1 and Q.sub.2. A series circuit of resistors R.sub.1 and R.sub.2 and a further resistor R.sub.3 are connected to the emitter of the power transistor Q.sub.1, and the base of a further transistor Q.sub.1 is connected to a connecting point between the resistors R.sub.1 and R.sub.2. The collector of the transistor Q.sub.3 is connected to the base of the power transistor Q.sub.1. When a signal having a great amplitude is applied to the base of the power transistor Q.sub.1, the voltage across the resistor R.sub.2 rises so that the transistor Q.sub.3 is put into an operative condition. Since part of base current of the power transistor Q.sub.1 is drawn into the transistor Q.sub.3, the base current of the power transistor Q.sub.1 is restricted so that the power transistor Q.sub.1 is released from an overload condition, thereby preventing junction breakdown of the power transistor Q.sub.1 by thermal runaway.
A series circuit of resistors R.sub.4 and R.sub.5 and a diode D.sub.1 are connected to the emitter of the power transistor Q.sub.1, and the base of another transistor Q.sub.5 is connected to a connecting point between the resistors R.sub.4 and R.sub.5 and the collector of the transistor Q.sub.5 is connected to a relay driving circuit 3, thereby constituting a short-circuiting detecting circuit. If short-circuiting takes place between an output of the audio frequency amplifier circuit 1 and the ground, then the transistor Q.sub.5 drives the relay driving circuit 5 to open a relay contact 4, thereby to cut the load to prevent excessive current from flowing through the power transistor Q.sub.1.
Further in order to protect the power transistors Q.sub.1 and Q.sub.2 from junction breakdown caused by abnormal heat generation from them arising from short-circuiting or reception of excessively high power, a thermistor 2 is attached to a heat sink or a casing on which the power transistors Q.sub.1 and Q.sub.2 are mounted. The thermistor 2 detects an abnormal temperature and drives the relay driving circuit 3 to open the relay contact 4, thereby to cut the audio frequency power amplifier circuit 1 from the load to prevent excessively high current from flowing through the power transistor Q.sub.1 to protect the power transistor Q.sub.1 from junction breakdown caused by thermal runaway.
If circuit constants of components of the protective apparatus for a power transistor such as a limiter circuit or a short-circuiting detecting circuit are set so that output power very near to a maximum rated power may be obtained, then operation of the protective apparatus may sometimes be delayed because of a delay in conduction of heat to the power transistor or the head sink and the protective apparatus may not function adequately. In such a case, an excessive load may be applied to the power transistor beyond its maximum collector loss Pcmax to cause breakdown of the junction of the power transistor. Further, since the protective apparatus is provided on the output stage of the audio frequency power amplifier circuit, the operation of the protective apparatus may be displaced from the phase of an output of the audio frequency power amplifier circuit so that unexpected muting may be applied, resulting in failure to obtain sufficiently high output power. On the other hand, if circuit constants are set such that the protective apparatus may be driven with an excessively high safety factor, then sufficiently high power cannot be obtained.
When the protective apparatus does not operate sufficiently and consequently the power transistor generates heat abnormally, conventionally a muting operation of the power transistor becomes effective only when the thermistor 2 operates after the heat sink is heated. In other words, there is a delay time by a thermal time constant until muting becomes effective, and for the delay time, the power transistor Q.sub.1 or Q.sub.2 may thermally run away to cause breakdown.
Further, in the case of an audio frequency power amplifier circuit for driving a loudspeaker for commercial use, if sound is interrupted by operation of the protective apparatus, then a suspensive or disagreeable feeling that a failure may have taken place may be given to listeners, customers or the like. Thus, it is required to prevent abnormal heat generation of a power transistor by excessive current without completely cutting an output stage of an audio frequency power amplifier circuit for a comparatively long period of time.